Homogeneously Dispersed Polyrotaxane in Epoxy Adhesive and Its

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Homogeneously Dispersed Polyrotaxane in Epoxy Adhesive and Its Improvement in the Fracture Toughness Sirawit Pruksawan,†,‡ Sadaki Samitsu,*,† Hideaki Yokoyama,§ and Masanobu Naito*,†,‡,§

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†

Research and Services Division of Materials Data and Integrated System (MaDIS), National Institute for Materials Science (NIMS), 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan ‡ Program in Materials Science and Engineering, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1, Tenodai, Tsukuba, Ibaraki 305-8571, Japan § Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Toudaikasiwakyanpasu, Kashiwanoha, Kashiwa-shi, Chiba 277-8561, Japan S Supporting Information *

ABSTRACT: Polyrotaxanes (PRs), a new class of supramolecular polymers, have recently attracted considerable attention in materials science because of their unique structure and intriguing effects on material properties. Here, we report that a PR is capable of toughening a rigid epoxy adhesive without phase separation morphology, unlike the interfacemediated toughening mechanisms established in conventional epoxy resins. A PR bearing polycaprolactone graft chains on wheel-like molecules was dispersed homogeneously in an epoxy adhesive via intermolecular hydrogen bonding. The PRincorporating epoxy adhesive exhibited simultaneous increase in adhesive strength, fracture displacement, and fracture toughness while retaining its high glass transition temperature and tensile modulus. Morphological, thermal, and mechanical characterizations suggested that the toughening mechanism originates from the PR supramolecular structure, allowing the wheel-like molecules to rotate around and slide along the polymer main chain. The study revealed the fracture behavior of PRcontaining epoxy adhesives, which may be beneficial for practical applications of network polymers.

1. INTRODUCTION Single-molecule machines, which mimic sophisticated biological functions in an artificial manner, have attracted considerable attention from a fundamental viewpoint in supramolecular chemistry since the 1960s.1 While various synthesis examples of molecular motifs have been developed, polyrotaxanes (PRs) have recently emerged as an interesting supramolecular polymeric motif in materials science.2,3 A PR consists of a linear polymer and wheel-like molecules, where the linear polymer threads all the wheel-like molecules and both its ends are capped with bulky end groups.4−7 Because the wheel-like molecules are constrained topologically within the linear polymer, they can rotate around and slide along the linear polymer chain without dethreading from it.8,9 The unique molecular structure of PRs provides additional degrees of freedom in molecular motion (i.e., rotation and sliding of wheel-like molecules), based on which various intriguing applications have been realized, including drug and gene delivery,10,11 tissue engineering,12 molecular electronics,13 electrochromic devices,14 and stimuli-responsive materials.15 Focusing on long-distance sliding motions of PRs, many studies have demonstrated the ability of PRs to improve the mechanical performance of soft rubbery materials, such as extremely stretchable hydrogels16,17 and elastomers,18,19 selfhealing materials,20 and highly elastic binders for batteries.21 In © XXXX American Chemical Society

contrast to the comprehensive studies on rubbery materials,19,22,23 only three studies have preliminarily reported the incorporation of PRs into hard glassy network polymers, such as epoxy resins. Wang et al. incorporated an unmodified PR into a novolac-type epoxy resin and examined its effect on the storage modulus and mechanical relaxation.24 Li et al. found that a PR modified with polycaprolactone (PCL) can facilitate miscibility into an epoxy resin by a specific intermolecular interaction, which enhances the impact strength of the bulk resin because of plastic deformation.25 Seo et al. reported a tertiary amine-functionalized PR as an accelerator that can simultaneously enhance the tensile strength, tensile strain, and impact strength of an epoxy resin.26 Although PRs have been shown to improve the mechanical properties of soft network polymers, their beneficial effects have not completely been revealed thus far in the case of glassy materials.27,28 To satisfy the recent demand for multimaterials in aircraft and automobile construction, it is necessary to further improve the mechanical performance of hard adhesives, which is another major application of epoxy resins.29 In general, the adhesive property is considerably different from the bulk Received: November 15, 2018 Revised: February 13, 2019

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DOI: 10.1021/acs.macromol.8b02450 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. (a) Chemical structure of polyrotaxane (PR) and (b) curing reaction of diglycidyl ether of bisphenol A (DGEBA) by 4,4′diaminodiphenylmethane (DDM). the PR and poly(ethylene glycol) (PEG), an axle polymer of the PR, were 400000 and 20000 g/mol, respectively.31 Previous studies have shown that the PR has ∼60 α-cyclodextrin rings on the PEG.27,32 The average molecular weight of the PCL graft chains on hydroxypropylated α-cyclodextrins was determined to be 1712 ± 114 g/mol using the procedure reported by Minato et al.19 The molecular weight, which was calculated based on the 1H NMR spectrum of PR (Figure S1), was consistent with the value suggested by Araki et al.18 PCL with a molecular weight of 2000 g/mol, obtained from Sigma-Aldrich (Japan), was used as a modifier for comparison with the PR. Note that the molecular weight of PCL is similar to that of the PCL graft chains of PR. Both PEG and hydroxypropylated α-cyclodextrins with molecular weights of 20000 and 1180 g/mol, respectively, were purchased from Sigma-Aldrich (Japan) and used as references. All the chemicals were used as received. The chemical structure of the PR is shown in Figure 1a. 2.2. Preparation and Curing of Adhesive Samples. A certain amount of the PR (0.1, 0.2, or 0.3 g) was added into the liquid DGEBA (1.0 g) and mixed at 90 °C for 30 min under magnetic stirring. Homogeneous blends of DGEBA containing 10, 20, and 30 parts per hundred resin (phr) of the PR were obtained because the waxy PR becomes fluidic at ∼90 °C. The weight ratio of DDM to DGEBA was set to 29:100, which corresponds to a stoichiometric ratio of the epoxy equivalent molecular weight of DEGBA and the amine equivalent molecular weight of DDM. Then, 0.29 g of powdered DDM was added directly to the blends and mixed at 90 °C for 20 min to achieve complete dissolution. The adhesive precursor was cured at 150 °C for 2 h, resulting in solid adhesives. The curing reaction of DGEBA by DDM is shown in Figure 1b. The PR-modified adhesives were abbreviated as PR-10 adhesive, PR-20 adhesive, and PR-30 adhesive, where the numbers correspond to the phr of the PR. Through the same procedure, PCL-modified adhesives were prepared using PCL instead of the PR, and these were abbreviated similarly to

properties mainly because of the constraint between rigid substrates.30 However, previous studies on the effect of PRs have reported limited mechanical properties only of the bulk material. In this study, we investigate the effect of PRs on adhesive properties. An adhesive matrix composed of bisphenol A epoxy monomer and an aromatic diamine as a curing agent was developed as a model system of a densely cross-linked rigid network polymer. Suitable chemical modifications with PCL graft chains on the wheel-like molecules enabled us to homogeneously disperse the PR into the epoxy adhesive. A thin layer of the PR-containing adhesive was fixed between aluminum substrates. The adhesive strength and fracture toughness of the adhesive were examined by a singlelap shear test and a double-cantilever beam (DCB) test, respectively. Various characterizations of the thermal properties, morphology, intermolecular interaction, and molecular relaxation and fracture surface morphology were performed, which suggested that the PR toughens the rigid epoxy adhesive through a unique toughening mechanism.

2. EXPERIMENTAL SECTION 2.1. Materials. Diglycidyl ether of bisphenol A (DGEBA), a conventional epoxy resin, was used as a matrix. DGEBA with an epoxide equivalent weight of 170.2 g/mol was obtained from SigmaAldrich (St. Louis, MO). As a curing agent, 4,4′-diaminodiphenylmethane (DDM) with an amine hydrogen equivalent weight of 49.6 g/mol was obtained from TCI Chemicals (Japan). A PR with PCL graft chains on α-cyclodextrins as the wheel-like molecules (grade name: SH2400P) was obtained from Advanced Soft Materials Inc. (Japan) and was used as a modifier.18 The molecular weights of B


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Macromolecules the PR-modified adhesives. Neat epoxy adhesive prepared without any modifier was denoted as EX adhesive. Single-lap shear and DCB specimens were fabricated in accordance with ASTM D1002 and D5528, respectively (Figure 2). Aluminum

Tensile tests of the bulk adhesive specimens were performed using a Shimadzu autograph AG-Xplus operated at a crosshead speed of 2 mm/min. In all the tensile tests, the specimen displacement was measured with respect to the sample gauge length. The tensile strains of the bulk adhesives were calculated from the gauge lengths of each sample. At least three bulk specimens were measured, and the average values were listed with their respective standard deviations. The tensile strengths of the bulk adhesives (σs) were calculated by dividing the maximum loads by the cross-sectional area of each bulk adhesive. DCB tests were conducted on the basis of ASTM D3433 using a Shimadzu autograph AG-Xplus operated at a crosshead speed of 0.5 mm/min. The crack length (a) was measured along the edge of the specimen using a Sony Starvis IMX178 high-resolution camera. The extension (δ) on the load line was measured as the displacement of the crosshead. To introduce an artificial crack, a specimen was initially loaded until the crack length reached ∼70 mm from the load line, and the specimen was then unloaded. The process for generating the artificial crack was not used for data analysis. The specimen was loaded until the crack had propagated ∼20 mm from the previous length. The test was paused for 5 min to cease the crack propagation, and the specimen was unloaded to 5 N. This procedure was repeated until specimen failure. The crack extension resistance (GI) was calculated on the basis of a modified compliance calibration method, and the fracture toughness (GIC) was defined as the maximum GI. At least three specimens were tested, and the results were averaged for each adhesive type. 1 H NMR spectra were recorded on a JEOL ECS-400 (400 MHz) in deuterated chloroform with tetramethylsilane as an internal standard. Differential scanning calorimetry (DSC) was performed using a Shimadzu DSC-60 Plus equipped with a liquid-nitrogen cooler. Thermogravimetric analysis (TGA) was conducted using a Netzsch STA 2500 Regulus. Transmission electron microscopy (TEM) and scanning transmission electron microscopy−energy-dispersive X-ray spectroscopy (STEM−EDS) were performed using a JEOL JEM2100F at an acceleration voltage of 200 kV. TEM and STEM-EDS were operated in the bright- and dark-field mode, respectively. Smallangle X-ray scattering (SAXS) measurements were performed at the beamline BL-6A in the Photon Factory of the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan. The details of the beamline setup are described elsewhere.33 An X-ray wavelength of 1.5 Å and a sample−detector distance of 2420 mm (calibrated using silver behenate) gave a detectable scattering vector (q) range of 0.04−2.3 nm−1. Data processing was performed using SAngler.34 Fourier-transform infrared spectroscopy (FTIR) was performed using a JASCO FT-IR 6100 operated in transmission geometry. Dynamic mechanical analysis (DMA) was performed using a TA Instruments RSA-G2 equipped with a liquid-nitrogen cooler. The fracture surface morphology of the DCB specimens was visualized by white-light confocal microscopy in height mode using a Lasertec OPTELICS HYBRID instrument. The fastest decay autocorrelation length (Sal) and texture aspect ratio (Str) of the height images were calculated according to the ISO standard.35 The detailed characterization techniques are described in the Supporting Information.

Figure 2. Schematic illustration of (a) single-lap shear specimen and (b) DCB specimen. substrates were surface treated following ASTM D2651. Specimens were prepared by spreading the adhesive precursor over a rectangular area and curing at 150 °C for 2 h in an oven. The adhesive thicknesses of the single-lap shear and DCB specimens were approximately 40 and 100 μm, respectively, unless otherwise specified. Two piano hinges were attached to the DCB specimen at the load line to apply mode I loads. The detailed preparation procedures for the adhesive specimens are described in the Supporting Information. Bulk specimens for tensile tests were prepared by casting an adhesive precursor into a dog-bone-shaped Teflon mold (35 mm × 5 mm × 1.5 mm), which was degassed at 90 °C for 15 min and cured at 150 °C for 2 h. 2.3. Adhesive Tests and Characterization Techniques. Lap shear tests of the adhesive specimens were performed using a Shimadzu autograph AG-Xplus, which was equipped with a 10 kN load cell and operated at a crosshead speed of 2 mm/min unless otherwise specified. The specimen displacement was measured precisely with respect to the sample gauge length by using a Shimadzu TRViewX digital video extensometer. To ensure reproducible data, at least five adhesive specimens were measured, and the average values were listed with their respective standard deviations. The adhesive strengths of the adhesives (σad) were calculated by dividing the maximum load before fracture by the defined overlapped area of each joint. The lap shear test was conducted at room temperature, 90, 135, and 150 °C using a chamber temperature controller. The specimens were equilibrated in the chamber at each testing temperature for 1 h before measurement.

3. RESULTS AND DISCUSSION 3.1. Adhesive Performance. 3.1.1. Adhesive Strength. The adhesive specimens were prepared by fixing a thin adhesive layer between aluminum substrates, followed by curing at 150 °C for 2 h. The adhesives received more than 99% cure, as validated by the DSC and FTIR results (for details, see Figure S2). The specimens were subjected to a single-lap shear test (Figure 3a); the average σad and fracture displacement (ΔLf) are summarized in Table 1. The shear stress of the EX adhesive increased with the shear displacement, and breakage finally occurred at ΔLf = 0.11 mm. The curve gave a maximum σad of 9.8 MPa. The abrupt fracture without a plateau region suggests the brittle behavior of the EX C


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Figure 3. (a) Plot of shear stress as a function of shear displacement measured by single-lap shear test and (b) plot of tensile stress as a function of tensile strain measured by tensile test: EX adhesive (black dotted), PR-20 adhesive (red solid), and PCL-20 adhesive (blue dashed). (c) Adhesive strength plotted as a function of modifier content: PR adhesive (red solid) and PCL adhesive (blue solid).

adhesive. The fracture surfaces of the specimens were covered with thin layers of adhesive. This indicates that fracture occurred in the internal part of the adhesive layer, not at the interface between the adhesive and the substrates, which is known as cohesive failure. Based on the cohesive fracture of the EX specimens, the mechanical properties of the bulk adhesive should be improved because the bond strength between the adhesive and the substrates is sufficiently high. The bulk mechanical properties of the adhesives were examined by a tensile test using a dog-bone-shaped specimen (Figure 3b and Figure S4); the results are summarized in Table 1. When 20 phr of the PR was incorporated into the epoxy resin, the tensile strength (σs) and fracture strain (τf) of the bulk adhesive were improved by 24% and 38%, respectively, while the tensile modulus (Es) was maintained. A previous study also found a significant increase in τf at a PR content of 30 phr; however, σs and Es decreased significantly.25 The improvement of the bulk mechanical properties by PR incorporation is expected to make a positive contribution toward the adhesive properties. As shown in Figure 3a, σad and

Table 1. Adhesive, Mechanical, and Thermal Properties of EX, PR-20, and PCL-20 Adhesives adhesive

EX

PR-20

PCL-20

σad (MPa)a ΔLf (mm)a GIC (J/m2)b σs (MPa)c τf (%)c Es (GPa)c E′ (GPa)d Tg (°C)e Td (°C)f

9.3 ± 0.7 0.11 ± 0.01 91.8 ± 12.8 43.1 ± 1.4 2.6 ± 0.6 2.22 ± 0.34 2.5 183.1 376.8

12.4 ± 1.1 0.21 ± 0.01 357.8 ± 73.4 53.5 ± 4.6 3.6 ± 0.6 2.17 ± 0.41 1.9 173.3 372.1

8.8 ± 0.7 0.12 ± 0.03 74.6 ± 8.3 49.0 ± 4.4 2.9 ± 0.6 2.19 ± 0.23 1.7 128.1 376.4

Adhesive strength σad and fracture displacement ΔLf measured by lap shear test. bFracture toughness GIC measured by DCB test. c Tensile strength σs, fracture strain τf, and tensile modulus Es measured by tensile test of bulk adhesives. dStorage modulus E′ measured by DMA of bulk adhesives. eGlass transition temperature Tg measured by DSC. fDegradation temperature Td measured by TGA. a

D


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Macromolecules ΔLf of the PR-20 adhesive increased by 2.8 MPa and 0.09 mm, respectively, compared with the EX adhesive. Note that the shear displacement of the joints is not directly related to the shear displacement of the adhesive that was used for the specimen in the case of the single-lap shear test geometry, as discussed briefly in Table S1. Another PR-20 specimen was prepared by using different sample thicknesses and substrate treatment. When it was extended more slowly (at a crosshead speed of 0.01 mm/min), a large plastic deformation was clearly found before fracture (Figure S3). The result confirmed the ability of the PR modifier to improve the ductility of the specimen. The PCL-20 adhesive exhibited no substantial change in σad and ΔLf; strictly speaking, σad decreased slightly. No improvement in the adhesive properties of the PCL-20 adhesive seemed to correlate with its bulk mechanical properties, as in the case of the EX adhesive (Table 1). The result supports the interpretation that the increase in σad and ΔLf of the PR adhesive originates not from the effect of the PCL chains but from the unique molecular structure of the PR. In addition, σad of the adhesives was plotted as a function of the modifier content (Figure 3c). Note that σad of the PR adhesives increased with the PR content, which was obvious at a PR content of 20 phr. The optimum modifier content was determined to be 20 phr because higher modifier contents did not show a significant improvement in σad. Indeed, σad of the PR-100 adhesive was nearly identical to that of the PR-20 adhesive under the same fabrication and testing conditions (Figure S3b). The PCL adhesives exhibited a slight decrease in σad over the experimental errors with respect to the PCL contents investigated. The definite increase in σad with the PR content confirmed the positive contribution of the molecular structure of the PR; i.e., it ruled out the effect of the PCL graft chains. 3.1.2. Fracture Toughness. The fracture toughness of the adhesives was examined by a DCB test at room temperature. The adhesives fixed between aluminum substrates showed cohesive failure, as in the case of the lap shear tests. A series of typical load−extension curves for the EX, PR-20, and PCL-20 adhesives exhibited similar behavior although the values of the load varied considerably (Figure 4a). The load increased linearly with the extension in the initial region where no crack propagation was confirmed by camera inspection. When the crack started to propagate slowly, the load reached a maximum value accompanied by some fluctuation. The load then decreased abruptly because of rapid crack propagation where the extension stopped and the load was released by reducing the extension. The load−unload cycles were repeated several times until complete fracture of the specimen occurred. Compared with the EX adhesive, the PR-20 adhesive exhibited higher maximum loads; on the other hand, the PCL-20 adhesive exhibited lower ones, qualitatively indicating the improved fracture toughness due to PR addition. The crack growth resistance curves (R-curves) were quantitatively reconstructed by plotting GI as a function of a according to the procedure described in Figure S5. The values for GIC were determined from the maxima of the plot shown in Figure 4b. The average GIC for the EX adhesive was determined to be 91.8 J/m2, which is consistent with the values in the literature36 and validates the procedure of our DCB tests. While the average GIC of PR-10 adhesive showed a slight increase, that of PR-20 adhesive presented a remarkable increase up to 357.8 J/ m2, which was 290% greater than that of the EX adhesive. The PR-30 adhesive exhibited an average GIC that was 200% higher

Figure 4. (a) Load−extension curves recorded in DCB tests at room temperature: EX adhesive (black dotted), PR-20 adhesive (red solid), and PCL-20 adhesive (blue dashed). Load−extension curves of other adhesives are presented in Figure S6. (b) Fracture toughness of the PR adhesives and other adhesives determined by the DCB tests.

than that of the EX adhesive, while the GIC value was slightly smaller than that of the PR-20 adhesive. This result provided direct evidence of the positive effect of incorporating the PR into the epoxy resin on the GIC. For further comparison, each molecular component of PR, i.e., PCL, PEG, and hydroxypropylated α-cyclodextrin, was incorporated into the epoxy adhesive at 20 phr content. PCL or PEG mixed with the epoxy adhesive presented a rather transparent appearance to the naked eye, which indicated their miscibility with the epoxy adhesive, at least at the macroscopic scale. On the other hand, the epoxy adhesive containing hydroxypropylated α-cyclodextrin had an opaque appearance and many granular particles could be observed by optical microscopy, suggesting its incomplete dissolution into the epoxy adhesive. In spite of their different miscibility, the average GIC values of all DCB adhesives ranged between 74.6 and 108.4 J/m2, which coincided with the GIC value of the EX adhesive (within experimental error). This result verified that the toughening effect of the PR did not originate from either of the molecular components of PR but instead from the unique molecular structure of PR and the capability of rotation and sliding motion of the wheel-like components. Although a previous E


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The PCL-20 adhesive exhibited a small endothermic peak at 38.4 °C with an enthalpy of 0.5 J/g, which is the same temperature as the melting temperature of the pure PCL (Figure S8). Macroscopic phase separation of the PCL slightly occurred in the epoxy matrix, which is proven directly by a TEM image, as described in the next section. The Tg of the PR and PCL adhesives is plotted as a function of the modifier content (Figure 5). All the adhesives

study reported that PR incorporation enhances the impact strength of the bulk epoxy resin,25 the GIC determined by slow crack propagation represents another feature of the toughness of epoxy adhesives because of different geometries (bulk and adhesive) and time domains (unstable and stable). Previous studies have proposed several toughening mechanisms of epoxy resins depending on the types of polymeric modifiers and solid fillers.29,37 Rubber-toughened epoxy resins were developed by incorporating rubbery polymers or particles in epoxy resins.38 Reaction-induced phase separation occurring in a curing process often forms a sea−island structure because the spherical rubbery domains are bounded in a matrix of the cross-linked epoxy network. The phase separation structure can effectively release the stress concentration during crack propagation because of stretching and tearing of the rubbery domains, thereby enhancing GIC. In spite of the improvement in GIC, the rubber-toughened epoxy resins sometimes suffer from a decrease in the glass transition temperature (Tg) and elastic modulus, which undesirably degrades their thermomechanical stability.39,40 Engineering thermoplastics toughen epoxy resins by forming inverse sea−island and bicontinuous structures when the loading amount of such thermoplastics is sufficiently high.41 The interconnected phases of the thermoplastics increase the GIC considerably because engineering plastics have high mechanical strength and Tg. Inorganic fillers can dissipate the stress concentration at a crack tip by generating a large number of microscopic voids at the filler− matrix interfaces.42,43 Local delamination of the interfaces facilitates plastic deformation in a plastic zone, which results in crack deflection and crack front pinning. These well-known mechanisms mainly originate from the multiphase morphology of polymer blends and composites. As described above, the PR-20 adhesive shows relatively large improvements in GIC comparable to other conventional toughening agents for DGEBA, such as nanoparticles (∼1.1−3 times)36 and thermoplastics (∼0.6−5 times);41 rubbers present the highest improvement (up to ∼30 times)37 among toughening agents for DEGBA epoxy resins. To understand the toughening mechanism of the PR-incorporating epoxy adhesive and to evaluate its thermomechanical stability, we performed various material characterizations using DSC, TEM, FTIR, and DMA. The characterizations determined the Tg, morphology, intermolecular interaction, and molecular relaxation of the adhesives; thus, they revealed the unique toughening mechanism specific to the molecular structure of the PR, as described in the following sections. 3.2. Characterization of Adhesives. 3.2.1. Thermal Analysis. A DSC profile of the EX adhesive exhibited no exothermic or endothermic peaks, but a shift of the baseline occurred at 183.1 °C (Figure S7), which corresponds to the glass−rubber transition of an amorphous adhesive at Tg. Owing to the PR addition, the Tg of the PR-20 adhesive was 10 °C lower than that of the EX adhesive because of the low Tg of the PR. In fact, the pure PR exhibited a Tg of −61.4 °C accompanied by an exothermic peak at −32.9 °C and an endothermic peak at 34.9 °C, attributed to cold crystallization and melting of the PCL graft chains, respectively (Figure S8). In addition to no glass transition at the Tg of the pure PR, the DSC peaks derived from cold crystallization and the melting of the pure PR disappeared, which indicates no macroscopic phase separation of the PR modifier. The PCL-20 adhesive exhibited a significant suppression of Tg by 55 °C even though the Tg of the pure PCL is nearly the same as that of the PR.

Figure 5. Glass transition temperatures of PR adhesives (red circles) and PCL adhesives (blue squares) plotted as a function of modifier content. The dotted lines are plotted according to the additivity rule.

investigated in this study show only one Tg in the DSC profiles. At a modifier content of 30 phr, the Tg of the PR-30 adhesive decreased by only 20.8 °C, whereas that of the PCL30 adhesive decreased significantly by 80.9 °C. Because the Tgs of the PR and PCL are similar and ∼240 °C lower than the Tg of the EX adhesive, the Tgs of the PR and PCL adhesives are supposed to be significantly lower than the Tg of the EX adhesive. However, it is interesting that the decrease in Tg was suppressed significantly in the case of the PR adhesives. Miscible polymer blends, i.e., binary blends with a single Tg value, usually exhibit positive or negative deviations from the Tg value determined by the additivity rules.44,45 According to the expression reported by Lu et al.,44 a negative χ parameter contributes to a positive deviation, whereas the change in the specific heat capacity at glass transition for each polymer component results in a negative deviation. The positive deviation for the PR adhesive, which has not been reported previously,24,25 could be attributed to a negative χ parameter. This negative χ parameter was likely a result of the weak specific intermolecular interactions of the PR with the epoxy network because hydrogen bonding between them was detected by mid-IR spectroscopy, as discussed in section 3.2.3. The PCL adhesive showed a negative deviation, which implied a negligibly small χ parameter. Although the χ parameters of PR and PCL adhesives have not been determined experimentally as yet, the difference may result from the efficiency of hydrogen-bonding interaction that is promoted by the rotation and sliding motion of PR. The thermal stability of the adhesives was analyzed using TGA (Figure S9). All the adhesives are thermally stable up to 370 °C because of encapsulation into a thermally stable epoxy matrix. The degradation temperature (Td) values obtained by TGA are summarized in Table 1. F


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Macromolecules 3.2.2. Structural Analysis. The morphology of the PR-20 and PCL-20 adhesives was observed via TEM after staining ultrathin specimens with RuO4 (Figure 6). A low-resolution

theory states that binary polymer blends linear polymers exhibit a stronger tendency for phase separation with an increase in their molecular weights48,49 when each χ parameter has the same value. Because PR has a molecular weight that is 200 times larger than that of PCL, the homogeneous dispersion of the former suggested a larger negative χ parameter, as implied by the positive deviation on the Tg curve. Another indication of the homogeneous dispersion of the PR may be the effect of a kinetic trap during the rapid curing process because the high molecular weight of the PR significantly increases the viscosity of the liquid precursor of the PR adhesive. Indeed, when increasing the PR content up to 50 phr and more, the TEM images exhibited dark gray particles embedded in a light gray bicontinuous structure (Figure S11). Considering the STEM−EDS result of the PCL-20 adhesive, the dark gray contrast resulting from a larger amount of RuO4 is attributed to the PR-rich region containing many PCL graft chains. The contrast becomes more evident with increasing PR content, the size of which ranges from 5 to 20 nm. It will be a challenge to completely reveal the effect of the supramolecular structure on the polymer blend miscibility in the future. The mesoscopic morphology of the PR incorporated in the epoxy adhesive was further examined via SAXS (Figure 7). The

Figure 6. TEM images of (a) PCL-20 adhesive and (b) PR-20 adhesive observed after staining with RuO4. Inset: low-resolution TEM images.

TEM image of the PCL-20 adhesive showed dark gray spherical domains dispersed in a light gray matrix (Figure 6a). STEM−EDS mapping analysis confirmed that the dark gray domains contained more RuO4 as they were higher in intensity on both the Ru and O elemental maps as compared with the uniform intensity on the C elemental map (Figure S10). Based on the capability of RuO4 staining and the low PCL content, the dark gray regions were assigned to PCL-rich domains. The PCL-rich domain exhibited 0.4−1.2 μm spheres, which is consistent with a macroscopic phase separation of the epoxy− PCL blends reported previously46,47 despite the smaller domain size and fewer domains. The weakly phase-separated morphology observed in this study probably results from the higher miscibility with the DDM-cured DEGBA network and/ or low molecular weight of PCL (Mn = 2000) because the previous reports studied the phase separation behavior of the DEGBA network cured with 4,4′-diaminodiphenyl sulfone (DDS) containing high-molecular-weight PCL (Mn = 80000). A low-resolution TEM image of the PR-20 adhesive shows uniform light gray contrast without domain morphology. Even when many specimens were observed thoroughly, the recorded images exhibited only line patterns made by a diamond knife via ultramicrotomy and gradual texture caused by wrinkles of the specimens. A high-resolution TEM image exhibited relatively uniform light gray contrast (Figure 6b), which implies homogeneous dispersion of the PR in the epoxy matrix. The significant difference in morphology between the PR-20 and PCL-20 adhesives is apparently unusual because the PR includes a large number of PCL chains that correspond to approximately 79−81 wt % .32 The well-known Flory−Huggins

Figure 7. SAXS intensity of EX, PR-20, PR-50, and PR-100 adhesives plotted as a function of scattering vector.

SAXS profile of the EX adhesive showed no spectral features, such as a peak or a shoulder, but exhibited a logarithmic increase in intensity with decreasing q at q < 0.2 nm−1. A similar SAXS profile has been reported for DGEBA epoxy resins cured by an amine and an anhydride.50 The PR-20 adhesive presented a featureless profile similar to that of the EX adhesive, clearly indicating no phase separation morphology at the length scale of the SAXS measurement, consistent with the TEM observation. Furthermore, the power law behavior of the SAXS intensity in the small q range determined the slope to be −4.3 ± 0.3 on the basis of a linear curve fitted on a logarithmic scale. The result suggested that the spatial fluctuation of the density over the distance scale was much larger than that probed by the SAXS measurement. The density fluctuation originated not from the phase separation of the PR but from the inhomogeneous network formation as a consequence of the cross-linking reaction of the epoxy resins because the same feature was found in both the EX and the PR-20 adhesives.51 In contrast to the PR-20 adhesive, the G


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confined circumstances in the densely cross-linked epoxy network structure, as detected by DMA, which is discussed in the next section. 3.2.4. Molecular Relaxation Analysis. In the DMA profile between −100 and 200 °C (Figure 9a), the storage modulus

SAXS profiles of the PR-50 and PR-100 adhesives showed a clear difference from that of the EX adhesive and had a shoulder in the q range of 0.2−1 nm−1, which was apparently consistent with the nanosegregation morphology at the high PR content observed in the TEM images. 3.2.3. Intermolecular Interaction Analysis. To investigate the origin of the good miscibility of the PR adhesives, the intermolecular interaction between the PR and the epoxy matrix was analyzed on the basis of the mid-IR spectra in the range of 1675−1825 cm−1, which corresponds to the peaks assigned to the carbonyl groups (Figure 8). In contrast to the

Figure 8. Mid-IR spectra of pure PR, PR-30 adhesive, PR-20 adhesive, PR-10 adhesive, and EX adhesive.

flat baseline for the EX adhesive, the spectrum of the pure PR shows several intense peaks because the PR includes many carbonyl groups on the short PCL chains that are grafted on the wheel-like molecules. The peaks at 1725 and 1731 cm−1 are attributed to stretching vibrations of the PCL in the crystalline and amorphous states, respectively.52,53 The result shows that pure the PR contains crystalline and amorphous states of the PCL side chains, consistent with the DSC result. For the PR adhesives, the crystalline peak disappeared completely whereas the peak that was derived from the amorphous state was preserved at the same position. This result is consistent with no enthalpy change for the PR adhesives, as confirmed by the DSC result. Interestingly, another band was identified at 1716 cm−1, which existed even for the lowest PR content (10 phr). A similar band has been observed for epoxy/PCL blends reported previously,54,55 which is attributed to the intermolecular hydrogen bonding between the carbonyl groups of the PCL and the hydroxyl groups of the epoxy network (CO···H−O). This result indicates that the PCL graft chains in the PR can interact with the epoxy network even though they are covalently bonded to the wheel-like molecules of the PR. Because of the reversible behavior of the hydrogen bonding, the rotation and sliding motions of the wheel-like molecules probably allow the PCL graft chains to interact with the hydroxyl group of the epoxy network even after the epoxy network structure was permanently fixed by the cross-linking reaction. The motions most likely facilitate local conformational relaxation even in

Figure 9. DMA profiles of EX, PR-10, PR-20, and PR-30 adhesives: (a) storage modulus (E′) and (b) loss tangent (tan δ).

(E′) of the EX adhesive suddenly decreased by nearly 2 orders of magnitude at 180 °C. The significant drop is assigned to the glass−rubber transition, the temperature of which was in good agreement with the Tg as assessed by DSC. The EX adhesive exhibited a high E′ of 2.5 GPa in the glassy state at 25 °C, which was reduced to 56 MPa in the rubbery state at 180 °C. The PR-20 adhesive had an E′ of 1.9 GPa at 25 °C, which was 0.6 GPa lower than that of the EX adhesive. The decrease in E′ was not as large as that observed sometimes in the case of rubber-toughened epoxy resins.39 The PR-20 adhesive exhibited a tan δ peak at 163.3 °C, which was 10 °C lower than the Tg assessed by DSC. However, the reductions in E′ and Tg were so small that they did not show a noticeable dependence on the variations in the PR content. The Tg of the PR adhesives fell within the temperature range of 164−180 °C, which is comparable with the Tg of the EX adhesive. This result suggests that PR addition up to 30 phr leads to no H


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Figure 10. Height images of the fracture surface recorded by white-light confocal microscopy: DCB specimens of the (a) EX and (b) PR-20 adhesives.

substantial change in E′ and Tg, which is important for the excellent thermomechanical stability of the PR adhesives. The loss tangent (tan δ) profiles of the EX adhesive presented two dispersion peaks related to the molecular relaxations of the epoxy network (Figure 9b). Whereas the sharp peak at 150−200 °C was derived from the α mode of the glass−rubber transition, the broad peak at −100 to 0 °C was attributed to the β mode.56,57 The low temperature of the β mode implies a small activation energy due to the local motion of a single segment compared with the α mode, which is involved with the cooperative motion of several segments. Despite a single Tg as confirmed by DSC, all the PR adhesives had an additional broad peak at 25 to 150 °C. However, both the PCL and EX adhesives had no peak in this temperature range. The increase in the PR content led to an increase in the peak magnitude; thus, the origin of the tan δ peak is attributed to the effect of the PR. The additional relaxation was located at intermediate temperatures between the α and β modes, which suggests a moderate activation energy for the molecular motion. Wang et al. investigated the effect of a PR on a novolac-based epoxy resin using a PR without modification of the wheel-like molecules and found an additional mode similar to our result.24 Regardless of the chemical modification of the PR and the type of epoxy resin used by Wang et al. and in our study, the similar results probably suggest that the origin of the peak is a unique PR structure, which allows for rotation and sliding motions of the wheel-like molecules. Wang et al. also reported that the epoxy resin that separately includes all the molecular components of the PR (PEG, α-cyclodextrins, and bulky end groups) does not exhibit the additional peak in its tan δ profile.24 Furthermore, the additional peak did not appear for the PR consisting of wheel-like molecules packed densely in a short PEG chain because of the absence of rotational and sliding motions of the wheel-like molecules.24 These results imply that the additional relaxation mode originates from the rotation and/or sliding motion of wheellike molecules in the PR. It should be noted that another study reported by Li et al. did not find the additional peak even though it used a PCL-grafted PR incorporated in a DGEBA system.25 The loss modulus (E″) profiles are shown in Figure S12. 3.3. Fracture Surface Morphology. The fracture surfaces of the DCB specimens were observed by white-light confocal microscopy (Figure 10). All fracture surface images were taken at the middle part of the specimen length. The EX adhesive showed an island-and-sea morphology with two height levels.

The high and low regions corresponded to thick and thin adhesive layers, respectively, because the average height of the high region was much smaller than the total thickness of the adhesive layer and the low region included several pinholes that were probably present on the surface of the aluminum substrate. The morphology ranged roughly over several hundreds of micrometers in size. The boundary between the high and low regions was distributed along the crack propagation direction to some extent. On the other hand, the PR-20 adhesive presented an island-and-sea morphology that was much smaller than the EX adhesive. The boundary between the island and the sea was spread in all directions, somewhat independent of the crack propagation direction. Because the islands were the higher regions for a part near the interface, the complete image displayed three height levels. To quantitatively analyze the surface morphology, the fastest decay autocorrelation length, Sal, and texture aspect ratio, Str, were calculated by digital image analysis. The Sal value of PR20 adhesive (8.1 ± 5.7 μm) was 8 times smaller than that of the EX adhesive (64.8 ± 12.5 μm), reflecting a finer fracture surface morphology. The Str value of the PR-20 adhesive was 0.6 ± 0.2, which was closer to unity as compared with that of the EX adhesive having anisotropic morphology (Str = 0.2 ± 0.1). All the images showed large number of crack deflections and branches, which were responsible for the larger fracture surface area of the PR-20 adhesive. The deflections and branches strongly disturbed the straight propagation of the primary crack and therefore led to a significant increase in the GIC value in the DCB test. 3.4. Toughening Mechanism of PR-Modified Adhesives. This study demonstrated the ability of a PR to improve adhesive properties in terms of adhesive strength and fracture toughness via single-lap shear and DCB tests. TEM and SAXS measurements confirmed that the PR-toughened epoxy adhesive had a uniform morphology because of the homogeneous dispersion of the PR into the epoxy matrix when the PR content was 20 phr or less. The uniform morphology is different from the multiphase morphology reported previously for other toughened epoxy resins. The interface structures found in the multiphase morphology have been considered as the main structural feature suitable for improving the fracture toughness, as described above (see section 3.1.2). Considering the uniform morphology of the PRtoughened epoxy adhesive, another toughening mechanism independent of the deformation of the interfaces can be proposed for the PR adhesives, which is possibly involved with I


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in σad at 150 °C is attributed to the low elastic modulus in the rubbery state of the PCL-20 adhesive. When tested at a temperature higher than Tg of the PCL-20 adhesive, ΔLf of the PCL-20 joint increased remarkably, which is consistent with the large τf in the case of rubbery behavior of the PCL-20 adhesive. Although ΔLf of the adhesive joint increases significantly because of the rubbery state of the PCL-20 adhesive, the rubbery state at the same time suffered from a severe reduction in σad. The result directly demonstrated the technological difficulty in improving ΔLf without decreasing σad for conventional toughening agents. The simultaneous improvement in ΔLf and σad for the PR modifier is noteworthy for the future development of strong tough adhesives made from polymer materials.

the unique molecular structure of the PR. The hydrogenbonding interaction between the PR and the epoxy network was detected by FTIR and DSC. The rotation and sliding motions of the PR allowed reversible formation of the hydrogen bonding by locally changing the intermolecular arrangement even when the PR was embedded in the rigid network structure of highly cross-linked epoxy resins. The DMA result showed that the PR adhesive exhibited molecular relaxation in the temperature range just below the glass transition temperature, while the EX and PCL adhesive showed no relaxation at these temperatures. The appearance of the molecular relaxation is probably correlated to the improvement of the adhesive strength and fracture toughness because both were found only in the case of the PR adhesives, and the magnitude of the relaxation increased with the PR content. Because the large plastic deformation was observed in the tensile test of the PR-20 adhesive, the PR-20 specimen can undergo large plastic deformation ahead of a crack tip and effectively relax the stress concentration in the course of crack propagation, leading to an increase in GIC. A flexible molecular motif covalently introduced into the epoxy network structure can increase the fracture toughness of the epoxy adhesive by facilitating plastic deformation of the epoxy network. Instead, the flexible network reduces both Tg, and E′ significantly.58,59 Interestingly, the incorporation of the PR improved σad and GIC without a substantial change in Tg, σs, and Es, which was confirmed by DSC, DMA, and tensile tests. The unique supramolecular structure of the PR is an intriguing feature that makes the PR an effective toughening agent for cross-linked epoxy resins. 3.5. Thermomechanical Stability of Adhesives. On the basis of the thermomechanical properties of adhesives, we examined σad by single-lap shear tests at different testing temperatures up to 150 °C (Figure 11). Although σad of the

4. CONCLUSIONS We reported the adhesive properties of a PR-incorporated rigid thermoset composed of bisphenol A epoxy monomer and an aromatic diamine as a curing agent. PCL-modified αcyclodextrin was used as a wheel-like molecule of the PR to improve its organosolubility, which enabled us to disperse the PR homogeneously into the epoxy matrix. The uniform dispersion of the PR was proven with a combination of direct observations by TEM as well as structural analysis with SAXS and thermal analyses with DSC. Significant improvements in the adhesive strength, fracture displacement, and fracture toughness were observed when the PR was incorporated into an epoxy adhesive, while Tg remained relatively unchanged. By contrast, no significant improvement was observed when PCL was used as a modifier. The DMA result suggested that the additional degree of freedom of the PR structure gives an additional peak in the tan δ profile, which facilitates the suppression of crack initiation and propagation. The fracture surface morphology of the PR adhesive revealed that the crack path was not straight and consisted of numerous crack deflections and branches, resulting in a significant increase in GIC. The results suggest that the improved properties are attributed mainly to the unique molecular structure of the PR rather than to the plasticizing effect of PCL. The toughening mechanism realized by the PR incorporation is different from the toughening mechanisms previously proposed for rubbery polymers, thermoplastics, and solid fillers because there are no domain structures or interfaces. The molecular motion of the PR seems to play a role in the toughening process. In terms of practical applications, simultaneous improvement in the adhesive strength and toughness is highly desired. On the basis of comprehensive thermal, morphological, and mechanical analyses, we propose PRs as effective modifiers for epoxy adhesives, as they provide good strength and toughness. Although various adhesive modifiers have been studied, supramolecular-based modifiers are not clearly understood because of their complicated structure. Such a modified epoxy resin system would contribute toward establishing the inherent mechanical/thermal/adhesive nature of advanced thermosetting resins.

Figure 11. Adhesive strength plotted as a function of temperature: EX adhesive (black solid), PR adhesive (red solid), and PCL adhesive (blue solid).

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PR-20 adhesive decreased gradually as the temperature increased, the PR-20 adhesive maintained a large σad that was at least similar to that of the EX joint even at 150 °C. In addition, σad of the PCL-20 adhesive decreased by 61% above 130 °C, whereas it decreased by 15% below 130 °C. The characteristic temperature of 130 °C corresponds with Tg of the PCL-20 adhesive (Table 1). Therefore, the significant drop

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(3) Mayumi, K.; Ito, K.; Kato, K. Polyrotaxane and Slide-Ring Materials, 1st ed.; Royal Society of Chemistry: Cambridge, 2015. (4) Harada, A.; Li, J.; Kamachi, M. The Molecular Necklace: A Rotaxane Containing Many Threaded α-Cyclodextrins. Nature 1992, 356 (6367), 325−327. (5) Gibson, H. W.; Bheda, M. C.; Engen, P. T. Rotaxanes, Catenanes, Polyrotaxanes, Polycatenanes and Related Materials. Prog. Polym. Sci. 1994, 19 (5), 843−945. (6) Takata, T. Polyrotaxane and Polyrotaxane Network: Supramolecular Architectures Based on the Concept of Dynamic Covalent Bond Chemistry. Polym. J. 2006, 38 (1), 1−20. (7) Wenz, G.; Han, B.-H.; Müller, A. Cyclodextrin Rotaxanes and Polyrotaxanes. Chem. Rev. 2006, 106 (3), 782−817. (8) Mayumi, K.; Ito, K. Structure and Dynamics of Polyrotaxane and Slide-Ring Materials. Polymer 2010, 51 (4), 959−967. (9) Bruns, C. J.; Stoddart, J. F. The Nature of the Mechanical Bond: From Molecules to Machines, 1st ed.; Wiley Publication: 2016. (10) Li, J. J.; Zhao, F.; Li, J. Polyrotaxanes for Applications in Life Science and Biotechnology. Appl. Microbiol. Biotechnol. 2011, 90 (2), 427−443. (11) Yu, G.; Yang, Z.; Fu, X.; Yung, B. C.; Yang, J.; Mao, Z.; Shao, L.; Hua, B.; Liu, Y.; Zhang, F.; et al. Polyrotaxane-Based Supramolecular Theranostics. Nat. Commun. 2018, 9 (1), 766. (12) Yui, N.; Ooya, T. Design of Polyrotaxanes as Supramolecular Conjugates for Cells and Tissues. J. Artif. Organs 2004, 7 (2), 62−68. (13) Cacialli, F.; Wilson, J. S.; Michels, J. J.; Daniel, C.; Silva, C.; Friend, R. H.; Severin, N.; Samorì, P.; Rabe, J. P.; O’Connell, M. J.; et al. Cyclodextrin-Threaded Conjugated Polyrotaxanes as Insulated Molecular Wires with Reduced Interstrand Interactions. Nat. Mater. 2002, 1, 160−164. (14) Ikeda, T.; Higuchi, M.; Kurth, D. G. From Thiophene [2]Rotaxane to Polythiophene Polyrotaxane. J. Am. Chem. Soc. 2009, 131 (26), 9158−9159. (15) Fujita, H.; Ooya, T.; Kurisawa, M.; Mori, H.; Terano, M.; Yui, N. Thermally Switchable Polyrotaxane as a Model of StimuliResponsive Supramolecules for Nano-Scale Devices. Macromol. Rapid Commun. 1996, 17 (8), 509−515. (16) Okumura, Y.; Ito, K. The Polyrotaxane Gel: A Topological Gel by Figure-of-Eight Cross-Links. Adv. Mater. 2001, 13 (7), 485−487. (17) Bin Imran, A.; Esaki, K.; Gotoh, H.; Seki, T.; Ito, K.; Sakai, Y.; Takeoka, Y. Extremely Stretchable Thermosensitive Hydrogels by Introducing Slide-Ring Polyrotaxane Cross-Linkers and Ionic Groups into the Polymer Network. Nat. Commun. 2014, 5, 5124. (18) Araki, J.; Kataoka, T.; Ito, K. Preparation of a “Sliding Graft Copolymer”, an Organic Solvent-Soluble Polyrotaxane Containing Mobile Side Chains, and Its Application for a Crosslinked Elastomeric Supramolecular Film. Soft Matter 2008, 4 (2), 245−249. (19) Minato, K.; Mayumi, K.; Maeda, R.; Kato, K.; Yokoyama, H.; Ito, K. Mechanical Properties of Supramolecular Elastomers Prepared from Polymer-Grafted Polyrotaxane. Polymer 2017, 128, 386−391. (20) Nakahata, M.; Mori, S.; Takashima, Y.; Yamaguchi, H.; Harada, A. Self-Healing Materials Formed by Cross-Linked Polyrotaxanes with Reversible Bonds. Chem. 2016, 1 (5), 766−775. (21) Choi, S.; Kwon, T.; Coskun, A.; Choi, J. W. Highly Elastic Binders Integrating Polyrotaxanes for Silicon Microparticle Anodes in Lithium Ion Batteries. Science 2017, 357 (6348), 279−283. (22) Murakami, H.; Baba, R.; Fukushima, M.; Nonaka, N. Synthesis and Characterization of Polyurethanes Crosslinked by Polyrotaxanes Consisting of Half-Methylated Cyclodextrins and PEGs with Different Chain Lengths. Polymer 2015, 56, 368−374. (23) Kato, K.; Hori, A.; Ito, K. An Efficient Synthesis of LowCovered Polyrotaxanes Grafted with Poly(ε-Caprolactone) and the Mechanical Properties of Its Cross-Linked Elastomers. Polymer 2018, 147, 67−73. (24) Wang, X. S.; Kim, H. K.; Fujita, Y.; Sudo, A.; Nishida, H.; Endo, T. Relaxation and Reinforcing Effects of Polyrotaxane in an Epoxy Resin Matrix. Macromolecules 2006, 39 (3), 1046−1052. (25) Li, X.; Kang, H.; Shen, J.; Zhang, L.; Nishi, T.; Ito, K. Miscibility, Intramolecular Specific Interactions and Mechanical

Detailed preparation of adhesive samples; detailed characterization techniques, 1H NMR spectrum of PR in CDCl3; DSC profiles and near-IR spectra of precursor of EX adhesive and adhesives cured at different temperatures; plot of shear stress as a function of shear displacement measured by single-lap shear test at room temperature; plots of tensile stress of PR-100 adhesive as a function of tensile strain measured at room temperature; compliance versus crack length curves and crack growth resistance curves for DCB specimens; load−extension curves of other adhesives recorded in DCB test at room temperature; DSC profiles of EX, PR20, and PCL-20 adhesives; DSC profiles of pure PCL and PR; TGA profiles of PR, PCL, EX adhesive, PR-20 adhesive, and PCL-20 adhesive; STEM−EDS elemental mapping images of PCL-20 adhesive; TEM images of PCL-20, PR-20, PR-50, and PR-100 adhesives observed after staining with RuO4; DMA profiles of loss modulus of EX, PR-10, PR-20, and PR-30 adhesives; comparison of shear modulus of bulk EX adhesive and slope of shear stress−displacement curve obtained from a single-lap shear test of the EX adhesive (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(S.S.) E-mail [email protected]. *(M.N.) E-mail [email protected]. ORCID

Sadaki Samitsu: 0000-0002-4139-1656 Hideaki Yokoyama: 0000-0002-0446-7412 Masanobu Naito: 0000-0001-7198-819X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We express our gratitude to Prof. Kazuaki Kato of the University of Tokyo for his insightful comments on the PR and to Dr. Tetsuya Matsunaga for his informative advice on the mechanical tests. We gratefully acknowledge Prof. Chiaki Sato and Prof. Yu Sekiguchi for their useful advice about the DCB test. We thank Ms. Junko Takaishi and Mr. Chen Xian NG for their support in conducting various experiments and Dr. Mizuki Tenjimbayashi for graphical illustrations. We are grateful to Prof. Izumi Ichinose of NIMS for his instrumental support on the DMA and to Dr. Atsuro Takai and Prof. Masayuki Takeuchi of NIMS for their instrumental support on the TGA. S.S. thanks the TEM station at NIMS for instrumental support in TEM and Ms. Ichie Koda for her generous support in the preparation and staining of ultrathin specimens for TEM. SAXS measurements were performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2018G019). S.P. acknowledges the Research Fellowship of the NIMS Junior Researcher (2017− 2018). This paper is partially supported by TIA Kakehashi.

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REFERENCES

(1) Kay, E. R.; Leigh, D. A. Rise of the Molecular Machines. Angew. Chem., Int. Ed. 2015, 54 (35), 10080−10088. (2) Araki, J.; Ito, K. Recent Advances in the Preparation of Cyclodextrin-Based Polyrotaxanes and Their Applications to Soft Materials. Soft Matter 2007, 3 (12), 1456−1473. K


Article

Macromolecules Properties of a DGEBA Based Epoxy Resin Toughened with a Sliding Graft Copolymer. Chin. J. Polym. Sci. 2015, 33 (3), 433−443. (26) Seo, J.; Yui, N.; Seo, J.-H. Development of a Supramolecular Accelerator Simultaneously to Increase the Cross-Linking Density and Ductility of an Epoxy Resin. Chem. Eng. J. 2019, 356, 303−311. (27) Kato, K.; Mizusawa, T.; Yokoyama, H.; Ito, K. Polyrotaxane Glass: Peculiar Mechanics Attributable to the Isolated Dynamics of Different Components. J. Phys. Chem. Lett. 2015, 6 (20), 4043−4048. (28) Kato, K.; Nemoto, K.; Mayumi, K.; Yokoyama, H.; Ito, K. Ductile Glass of Polyrotaxane Toughened by Stretch-Induced Intramolecular Phase Separation. ACS Appl. Mater. Interfaces 2017, 9 (38), 32436−32440. (29) Kinloch, A. J. Toughening Epoxy Adhesives to Meet Today’s Challenges. MRS Bull. 2003, 28 (6), 445−448. (30) Abbott, S. Adhesion Science: Principles and Practice, 1st ed.; DEStech Publications: 2015. (31) Product Information of Advanced Softmaterials Inc.; http:// www.asmi.jp/en/product (accessed Jan 13, 2018). (32) Noda, Y.; Hayashi, Y.; Ito, K. From Topological Gels to SlideRing Materials. J. Appl. Polym. Sci. 2014, 131 (15), 40509. (33) Takagi, H.; Igarashi, N.; Mori, T.; Saijo, S.; Ohta, H.; Nagatani, Y.; Kosuge, T.; Shimizu, N. Upgrade of Small Angle X-Ray Scattering Beamline BL-6A at the Photon Factory. In AIP Conference Proceedings; American Institute of Physics: 2016; Vol. 1741, p 30018. (34) Shimizu, N.; Yatabe, K.; Nagatani, Y.; Saijyo, S.; Kosuge, T.; Igarashi, N. Software Development for Analysis of Small-Angle X-Ray Scattering Data. In AIP Conference Proceedings; American Institute of Physics: 2016; Vol. 1741, p 50017. (35) ISO 25178-2. Geometrical Product Specifications (GPS) Surface Texture: Areal - Part 2: Terms, Definitions and Surface Texture Parameters; International Organization for Standardization, 2012. (36) Domun, N.; Hadavinia, H.; Zhang, T.; Sainsbury, T.; Liaghat, G. H.; Vahid, S. Improving the Fracture Toughness and the Strength of Epoxy Using Nanomaterials − a Review of the Current Status. Nanoscale 2015, 7 (23), 10294−10329. (37) Zotti, A.; Zuppolina, S.; Anna, B.; Zarrelli, M.; Borriello, A. Fracture Toughening Mechanisms in Epoxy Adhesives. In Adhesives Applications and Properties; Rudawska, A., Ed.; IntechOpen: London, 2016; Vol. 2, pp 237−269. (38) Bagheri, R.; Marouf, B. T.; Pearson, R. A. Rubber-Toughened Epoxies: A Critical Review. Polym. Rev. 2009, 49 (3), 201−225. (39) Tripathi, G.; Srivastava, D. Effect of Carboxyl-Terminated Poly(butadiene-Co-Acrylonitrile) (CTBN) Concentration on Thermal and Mechanical Properties of Binary Blends of Diglycidyl Ether of Bisphenol-A (DGEBA) Epoxy Resin. Mater. Sci. Eng., A 2007, 443 (1), 262−269. (40) Ratna, D. Phase Separation in Liquid Rubber Modified Epoxy Mixture. Relationship between Curing Conditions, Morphology and Ultimate Behavior. Polymer 2001, 42 (9), 4209−4218. (41) Hodgkin, J. H.; Simon, G. P.; Varley, R. J. Thermoplastic Toughening of Epoxy Resins: A Critical Review. Polym. Adv. Technol. 1998, 9 (1), 3−10. (42) Marouf, B. T.; Mai, Y. W.; Bagheri, R.; Pearson, R. A. Toughening of Epoxy Nanocomposites: Nano and Hybrid Effects. Polym. Rev. 2016, 56 (1), 70−112. (43) Johnsen, B. B.; Kinloch, A. J.; Mohammed, R. D.; Taylor, A. C.; Sprenger, S. Toughening Mechanisms of Nanoparticle-Modified Epoxy Polymers. Polymer 2007, 48 (2), 530−541. (44) Lu, X.; Weiss, R. A. Relationship between the Glass Transition Temperature and the Interaction Parameter of Miscible Binary Polymer Blends. Macromolecules 1992, 25 (12), 3242−3246. (45) Kalogeras, I. M.; Brostow, W. Glass Transition Temperatures in Binary Polymer Blends. J. Polym. Sci., Part B: Polym. Phys. 2009, 47, 80−95. (46) Chen, J.-L.; Chang, F.-C. Phase Separation Process in Poly(εCaprolactone)-Epoxy Blends. Macromolecules 1999, 32 (16), 5348− 5356.

(47) Chen, J.-L.; Chang, F.-C. Temperature-Dependent Phase Behavior in Poly(ε-Caprolactone)-Epoxy Blends. Polymer 2001, 42 (5), 2193−2199. (48) Utracki, L. A. Thermodynamics of Polymer Blends. In Polymer Blends Handbook; Utracki, L. A., Ed.; Springer: Dordrecht, 2014; pp 123−201. (49) Mezzenga, R.; Boogh, L.; Månson, J.-A. E. A Thermodynamic Model for Thermoset Polymer Blends with Reactive Modifiers. J. Polym. Sci., Part B: Polym. Phys. 2000, 38 (14), 1893−1902. (50) Matyi, R. J.; Uhlmann, D. R.; Koutsky, J. A. Structure of Glassy Polymers. VII. Small-Angle X-Ray Scattering from Epoxy Resins. J. Polym. Sci., Polym. Phys. Ed. 1980, 18 (5), 1053−1063. (51) Dušek, K. Are Cured Thermoset Resins Inhomogeneous? Angew. Makromol. Chem. 1996, 240 (1), 1−15. (52) Cholake, S.; Mada, M.; Raman, R.; Bai, Y.; Zhao, X.-L.; Rizkalla, S.; Bandyopadhyay, S. Quantitative Analysis of Curing Mechanisms of Epoxy Resin by Mid- and near-Fourier Transform Infra Red Spectroscopy. Def. Sci. J. 2014, 64 (3), 314−321. (53) Li, L.; Wu, Q.; Li, S.; Wu, P. Study of the Infrared Spectral Features of an Epoxy Curing Mechanism. Appl. Spectrosc. 2008, 62 (10), 1129−1136. (54) Garton, A. Competitive Equilibria in Miscible Polymer Blends and Low Molecular Weight Analogues: A Fourier Transform Infrared Study. Polym. Eng. Sci. 1983, 23 (12), 663−668. (55) Clark, J. N.; Daly, J. H.; Garton, A. Hydrogen Bonding in Epoxy Resin/poly(ε-Caprolactone) Blends. J. Appl. Polym. Sci. 1984, 29 (11), 3381−3390. (56) Takahama, T.; Geil, P. H. The β Relaxation Behavior of Bisphenol-Type Resins. J. Polym. Sci., Polym. Phys. Ed. 1982, 20 (11), 1979−1986. (57) Grillet, A. C.; Galy, J.; Gérard, J.-F.; Pascault, J.-P. Mechanical and Viscoelastic Properties of Epoxy Networks Cured with Aromatic Diamines. Polymer 1991, 32 (10), 1885−1891. (58) Urbaczewski-Espuche, E.; Galy, J.; Gerard, J.-F.; Pascault, J.-P.; Sautereau, H. Influence of Chain Flexibility and Crosslink Density on Mechanical Properties of Epoxy/amine Networks. Polym. Eng. Sci. 1991, 31 (22), 1572−1580. (59) Yang, G.; Fu, S. Y.; Yang, J. P. Preparation and Mechanical Properties of Modified Epoxy Resins with Flexible Diamines. Polymer 2007, 48 (1), 302−310.

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Homogeneously Dispersed Polyrotaxane in Epoxy Adhesive and Its

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